For the replacement of individual missing teeth, a paradigm shift is currently observed with immediate implant placement and/or restoration of incisor, canine and premolar sites of the aesthetic zone. In molar sites however, anatomical, occlusal and biomechanical considerations that are deterrent factors influencing positive outcomes of this treatment. Current systematic reviews and meta-analyses have clearly identified a lack of properly designed controlled clinical trials comparing the outcome of a combined immediate implant placement and loading protocol with that of delayed placement in the molar region. Recently, a novel implant design with moderately roughened surface was introduced to allow a safe and predictable immediate implant placement in molar extraction sockets. The implant has a strongly tapered body with a diameter of 8 or 9 mm, grit-blasted surface and a built-in platform switched implant-abutment connection. The wide-diameter implant body was proposed to be advantageous for engaging the molar socket walls during surgery, enhancing primary stability, minimizing the need for augmentation procedures, and facilitating immediate restoration/loading.

This doctoral thesis aims to evaluate the outcome of this novel implant design when used for immediate restoration of single missing mandibular molar teeth. In a controlled clinical trial, an 8 or 9 mm diameter implant was placed into either a fresh molar extraction socket or a healed site. All the implants received provisional crowns within 48 hours. The provisional crowns were replaced with full ceramic crowns on zirconia abutments after 8 weeks. The overall implant success rate after one year of service for 24 implants in two treatment groups was 75.0%. Success rates were 83.3% for the delayed placement group and 66.7% for the immediate placement group respectively, with no significant difference observed between the two groups (P = 0.35).

The bone level changes around the implants were favourable after one year, with marginal bone gain of 0.41 ± 0.57 mm for the immediately placed implants and 0.04 ± 0.46 mm for the delayed placed implants. No statistically significant differences were noted between the two placement groups (P = 0.14), and the two platform switching designs used in this study (P = 0.68).

Implant stability measurements using resonance frequency analysis (RFA) identified the immediately placed implants to be more stable immediately after surgery (80.08 ± 5.07 ISQ) than the delayed placed implants (78.83 ± 3.69 ISQ). In contrast, after 8 week healing time, the implant stability measurements showed a higher ISQ values for the delayed placed implants (76.75 ± 10.24 ISQ) compared with those placed in fresh extraction sockets (75.70 ± 10.52 ISQ). The delayed placed implants were also more stable after one year (83.90 ± 2.51 vs. 76.63 ± 10.65). The implant stability measurements after eight weeks showed a better accuracy in predicting implants that were at risk of failure than those taken at the time of implant placement.

The influence of the insertion torque on the amount and distribution of stresses around wide-diameter implants placed in mandibular molar extraction sockets was investigated using four finite element models. Torque values of more than 50 Ncm were found to result in unfavourable stresses and high bone compression that may lead to necrosis and early implant failure.

Chipping of the ceramic veneers of the definitive all-ceramic crowns was the most common prosthodontic complication. The disto-lingual cusps of six crowns were found to be chipped after one year. The cooling rate and the inadequate support for the ceramic veneers during casting of the crowns were identified as the main reasons for failure.

The rehabilitation of single missing mandibular molars by immediately-placed and restored wide-diameter implants was associated with a relatively high failure rate. This finding is to be weighed against the advantages of reduced treatment time and no requirements for bone grafting procedures. Further research to optimize the design of oral implants for immediate placement and/or restoration in molar sites is still mandatory.